1. Field of the Invention
This invention relates generally to multi-user information systems and particularly to systems having mass information storage media formed of one or more arrays of individual memory units such as disk drives which service real time applications such as video on demand.
2. Description of the Related Art
The rapid advances in information system technologies have facilitated great improvement in the distribution of information and entertainment material. Such distribution has been aided by the development and rapid increase of information networks such as those provided by cable television systems or the like. One of the most promising information system developments in the entertainment industry is the availability of so-called "video on demand" for television viewers. The basic concept of video on demand is relatively simple and clearly attractive to consumers. The basic idea of video on demand provides that individual consumers or viewers are able to independently access the stored data source of a plurality of stored entertainment programs such as movies or the like. This independent access frees the individual consumers from the restrictive nature of scheduled network distribution of present day cable systems. Thus, the viewer in a video on demand system is able to simply "dial up" the media storage and distribution facility through the cable network and view the desired program at any time.
While the basic concept of video on demand is simple, its implementation in a practical environment is extremely difficult and complex. In essence, the system must be capable of providing each viewer with independent access to the stored entertainment material. In a typical cable system environment, thousands and perhaps tens of thousands of viewers are serviced. To provide each with independent access to the plurality of program materials within the massive media inventory is a daunting task. To further exacerbate the problem, the nature of entertainment material, namely video and audio information, represents a large amount of information to be communicated. Thus, an effective video on demand system requires that an immense quantity of program information be stored within the mass media and a great number of subscribers be able to access the stored mass media simultaneously or nearly simultaneously.
The need for high volume, high speed access to large media storage systems is not, however, unique to video on demand operations. In related uses of such information systems such as interactive video, the rapid storage and retrieval of data and/or information from the mass media is necessary. In addition, other such systems are applicable to interactive television and television broadcasting operations. Further movie industry uses such as special effects production and post-production processes also require rapid storage and/or retrieval of large amounts of data.
Powerful computing systems also require high speed access to large data storage systems. Despite their similar need for high speed, high bandwidth access to large media storage systems, video on demand systems and video servers operate in a substantially more demanding and difficult environment than other computing systems. Video servers must provide simultaneous access to a large number of viewers or users. In addition, this access must be rapid and provide speed and continuity approaching real-time access and real-time data flow. Unlike computer systems which are able to wait for data, video on demand and video servers "crash" if data is not timely available. In other words, video servers must provide predictable or, so-called, "deterministic" bandwidths within each data channel. The provision of this deterministic bandwidth within video servers and the continuous supply of timely data is a crucial element of system performance.
In attempting to provide the necessary bandwidth, information storage and information retrieval at high speed to multiple users, practitioners in the video server arts have resorted to ever faster processors and parallel processing with somewhat limited success. However, faster processors and parallel processing alone have not provided a complete solution to the specialized problems of video servers. Certain limitations in system performance arise out of the characteristics of memory devices themselves.
Most, if not all, systems requiring storage and retrieval of large amounts of data, utilize one or more randomly accessible memory devices such as disk drive memories. Disk drives are highly effective in such applications due to the speed with which they operate and substantial storage capacity which they exhibit. Thus, disk drives provide random access, substantial capacity and relatively rapid storage or retrieval of information. Typically, to provide greater memory, disk drives are arranged in large arrays which operate under the coordination and interface of one or more so-called disk controllers. As good as such disk arrays and disk controllers are, their operating characteristics do impose limitations on system performance.
System response speed and ability to maintain continuous, reliable and deterministic channel bandwidths is determined, in part, by the speed with which the system disk drives themselves operate.
Basically, to retrieve data, the disk drive must first locate it. Thus, a time interval is required for the disk drive head to locate the particular portion of the disk upon which data is stored. This process essentially involves moving the disk drive head to the appropriate disk radius (usually called "seek") and thereafter rotating the disk until the desired disk portion is proximate to the drive head (usually called "latency"). The actual time of this interval is a characteristic of the particular disk drive fabrication and varies between disk drive designs. However, all disk drives exhibit a data location time interval characteristic. This data location time interval represents "lost time" in systems such as video servers in that no data is retrieved or stored during this interval. The overall effect of such lost time is cumulative and directly reduces system bandwidth and speed.
There remains therefore a continuing need in the art for improved video server systems which maintain continuous, reliable and deterministic channel bandwidths while using plural arrays of disk memories and disk controllers as for data storage. There remains a further need for improved video server systems which reduce the effect of disk drive data location time on system bandwidths.